X-ray image intensifier tube and method of making same

ABSTRACT

THE PRESENT INVENTION RELATES IN GENERAL TO METHODS FOR MAKING PICK-UP SCREENS FOR X-RAY IMAGE INTENSIFIER TUBES AND, MORE PARTICULARLY, TO AN IMPROVED METHOD WHEREIN THE X-RAY FLUORESCENT PHOSPHOR SCREEN ELEMENT IS FORMED BY EVAPORATION OF AN ALKALI METAL HALIDE MATERIAL IN VACUUM AND CONDENSING THE EVAPORATED MATERIAL ON AN X-RAY TRANSPARENT PORTION OF THE X-RAY INTENSIFIER TUBE, WHEREBY A CURVED X-RAY IMAGE PICK-UP SCREEN IS FORMED WHICH HAS IMPORVED QUANTUM EFFICIENCY AND RESOLUTION. SUCH IMPROVED X-RAY IMAGE INTENSIFIER TUBES ARE ESPECIALLY USEFUL FOR, BUT NOT LIMITED IN USE TO, X-RAY SYSTEMS AND FOR INTENSIFYING GAMMA RAY IMAGES OBTAINED IN APPLICATIONS OF NUCLEAR MEDICINE.

March 5, 1974 w. E. SPICER 3.7955531 X'RAY IMAGE INTENSIFIER TUBE ANDMETHOD OF MAKING SAME Original Filed Dec. 27. 1966 HIGH VOLTAGE l4 PRIORART X-RAY comma GENERATORL Y\ X-RAY \\[\BE\AM VIEWING- SCREEN v5 FIG.2PRIOR ART 5 H63 My /j g \\w 2 INVENTOR.

WILLIAM E. SPICER PUMP FIG.4

. -0RNEY United States Patent ()flice 3,795,531 Patented Mar. 5, 1974Int. Cl. H013 31/49 US. Cl. 11733.5 C 15 Claims ABSTRACT OF THEDISCLOSURE The present invention relates in general to methods formaking pick-up screens for X-ray image intensifier tubes and, moreparticularly, to an improved method wherein the X-ray fluorescentphosphor screen element is formed by evaporation of an alkali metalhalide material in vacuum and condensing the evaporated material on anX-ray transparent portion of the X-ray intensifier tube, whereby acurved X-ray image pick-up screen is formed which has improved quantumefiiciency and resolution. Such improved X-ray image intensifier tubesare especially useful for, but not limited in use to, X-ray systems andfor intensifying gamma ray images obtained in applications of nuclearmedicine.

RELATED CASES The present application is a continuation application ofcopending parent application 606,514 filed Dec. 27, 1966 and nowabandoned and assigned to the same assignee as the present application.

Heretofore, X-ray image pick-up screens for X-ray image intensifiertubes have been made by settling phosphor particles out of a liquidslurry onto an X-ray transparent spherical dish, as of aluminum, formingthe pick-up face of the evacuated image intensifier tube. While suchtechniques are suitable for ZnS pick-up screen materials they aregenerally unsuited for producing alkali metal halide screens whichshould provide improved X-ray quantum efficiencies. Moreover, theparticulated screen produced by such settling methods has only abouthalf the density of the bulk material and has poorer resolution thanthat ohtainable from a screen having a higher density of the screenmaterial. Also it would be desirable to use a screen material havinghigher stopping power and quantum conversion efiiciency such as thatprovided by the alkali metal halide.

While it may be possible to form the pick-up screen of a thin slab ofalkali metal halide, which has been deformed to produce the sphericalshape, such a deformation of the slab of phosphor material may seriouslydegrade the conversion efficiency and, thus, resolution of the convertedX- ray image because of the plastic deformation of the alkali halide.

In the present invention, the spherical pick-up screen is formed byevaporation of an alkali halide material, such as CsI, KI, NaI, Rb-I,CsBr, or LiI, in vacuum onto the inside of the spherical X-raytransparent pick-up face plate of the image intensifier tube. Such anevaporated pick-up screen has a density which is approximately equal tothat of the bulk material and, therefore, will provide enhancedresolution and quantum conversion efficiency.

'In one embodiment of the present invention, the alkali halide phosphorscreen material is co-evaporated with its activator material either byevaporation of an activated alkali metal halide or by simultaneousevaporation of the alkali metal halide and its activator.

In another embodiment of the present invention, the alkali halide screenis evaporated and condensed in place and subsequently activated bycoating the screen with the activator, as by evaporation, and thendiffusing the activator from the coating into the screen material.

The principal object of the present invention is the provision ofmethods for making improved X-ray image intensifier tubes.

One feature of the present invention is the provision of a method formaking the X-ray pick-up screen of an image intensifier tube wherein analkali metal halide screen material is evaporated in vacuum onto anX-ray transparent substrate, whereby a phosphor screen is produced whichhas improved resolution and quantum conversion efficiency.

Another feature of the present invention is the same as the precedingwherein the alkali halide material is coevaporated with its activatormaterial.

Another feature of the present invention is the same as the precedingfeature wherein the alkali halide to be evaporated includes itsactivator.

Another feature of the present invention is the same as the firstfeature wherein the alkali halide material and its activator materialare simultaneously evaporated from separate sources onto the substratemember.

Another feature of the present invention is the same as the firstfeature wherein the alkali halide screen, as deposited on the substrate,is activated by coating the surface of the screen with an activatormaterial and then dilfusing the activator into the alkali halide screenby raising the screen to an elevated temperature.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the alkali metal halide screen isannealed to remove minute residual plastic deformation of the material.

Other features and advantages of the present invention will becomeapparent upon a persual of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic line diagram of an X-ray image intensifier tube ofthe prior art,

FIG. 2 is an enlarged cross sectional view of a portion of the structureof FIG. 1 delineated by line 22,

FIG. 3 is a view similar to that of FIG. 2 depicting the pick-up screenconstruction of the present invention,

FIG. 4 is a schematic line diagram of an apparatus for evaporatingalkali halide materials in vacuum, and

FIG. 5 is an alternative evaporator apparatus to that depicted in FIG.4.

Referring now to FIGS. 1 and 2, there is shown a prior art X-ray system1 employing an X-ray image intensifier tube 2. Such a system isdescribed in an article entitled, X-ray Image Intensification With aLarge Diameter Image Intensifier Tube, appearing in the American Journalof Roentgenology Radium Therapy and Nuclear Medicine, volume 85, pages323-341 of February 1961. Briefly, an X-ray generator 3 serves toproduce and direct a beam of X-rays onto an object 4 to be X-rayed. Theimage intensifier tube 2 is disposed to receive the X-ray image of theobject 4.

The image intensifier tube 2 includes a dielectric vacuum envelope 5 asof glass approximately 17 inches long and 10 inches in diameter. Thepick-up face portion 6 of the tube 2 comprises a spherical X-raytransparent portion of the envelope 5, as of aluminum or conductiveglass, which is operated at cathode potential. An image pick-up screen 7of X-ray sensitive particulated phosphor such as ZnS is coated onto theinside spherical surface of the envelope portion 5 to a thickness as of0.020". A chemically inert optically transparent buffer layer 8 iscoated over the phosphor layer 7. A photo-cathode layer 9 is formed overthe buffer layer 8.

In operation, the X-rays penetrate the object 4 to be observed. Thelocal X-ray attenuation depends on both the thickness and atomic numberof the elements forming the object under observation. Thus, theintensity pattern in the X-ray beam after penetration of the object 4contains information concerning the structure of the object. The X-rayimage passes through the X-ray transparent envelope section 5 and fallsupon the X-ray sensitive phosphor layer 7 wherein the X-ray photons areabsorbed and re-emitted as optical photons, typically in the bluefrequency range. The optical photons pass through the transparentbufifer 8 to the photo-cathode 9 wherein they produce electrons. The.electrons are emitted from the photo-cathode in a pattern or imagecorresponding to the original X-ray image. The electrons are acceleratedto a high velocity, as of 30 kv., within the tube 2 and are focusedthrough an anode structure 12 onto a fluorescent screen 13 for viewingby the eye or other suitable optical pick-up device. Electron focusingelectrodes 14 are deposited on the interior surfaces of the tube 2 tofocus the electrons through the anode 12.

In the intensifier tube, one 50 kev. photon of X-ray energy absorbed bythe X-ray sensitive pick-up screen produces about 2000 photons of bluelight. These 2000 photons of blue light produce about 400 electrons whenabsorbed in the photo-emitter layer 9. The 400 electrons emitted fromthe photo-cathode produce about 400,000 photons of light in the visibleband when absorbed by the fluorescent viewing screen 13. Thus, the X-rayimage is converted to the visible range and greatly intensified forviewing.

One of the problems with the prior art intensifier tube 2 is that theparticulated pick-up screen has less than optimum resolution due to thefact that the particulated material has about one-half the density ofthe material in bulk form. Thus, to provide a certain probability ofstopping or absorbing an X-ray photon, the particulated layer 7 musthave about twice the thickness of such a layer if it had bulk density.The thicker the layer 7 the poorer its X-ray resolution. Moreover, theparticulated material serves to scatter the emitted optical photons,thereby still further reducing resolution. I

In addition, it is desirable to utilize a pick-up screen material havinga greater intrinsic stopping or absorbing power for X-rays. Suchimproved materials include the alkali metal halides such as, forexample, CsI, KI, NaI, RbI, CsBr, and LiI. These improved materials suchas CsI and NaI are obtainable in bulk slab form'from Harshaw ChemicalCompany of Cleveland, Ohio. However, when they are distorted from theslab form into the spherical slab form, to conform to the sphericalpick-up face 6 of the image intensifier tube 2, it is expected that theconversion efficiency and, hence, resolution of the converted X-rayimage is deleteriously affected.

Referring now to FIGS. 35 there is shown a section of the X-ray pick-upscreen formed in accordance with the methods of the present invention.More particularly, the alkali halide pick-up screen layer 16 is formedon the spherical X-ray transparent substrate member 5 by evaporation inVacuum.

In a first method, the substrate member 5 is cleaned and disposed in avacuum chamber 17 of a vacuum evaporator 18. A crucible 19 containingthe activated alkali metal halide phosphor 21 in bulk form is heated toa temperature sufiicient to evaporate the phosphor material as by anelectrical heating element 22. The evaporated activated alkali halide iscondensed (deposited) on the substrate 5 to the desired thickness as of0.010" for an X-ray image intensifier and to 0.060" for a gamma rayintensifier. As used herein, X-ray is defined to include X-rays andother high energy radiation including gamma ray radiation.

The bulk activated alkali halide may include any one of a number ofdifferent activators to render the pick-up screen 16 fluorescent uponabsorption of X-rays at room temperature. For example, CsI may includeTiI or NaI,

, Na or LiI as activators.

After the screen layer 16 has been deposited it is preferably annealedto remove any residual minute plastic deformations thereof because suchdeformations have an adverse effect upon quantum conversion efiiciency.A suitable annealing process is to heat the screen 16 as by heater 23 invacuum to within 10 C. of the melting point of the screen material for0.5 to 2.0 hours and then cool the screen 16 through to 400 C. in 10hours and then cool to room temperature in another 10 hours.

An ultra clean vacuum pump 24 is connected into the evaporation chamber17 to maintain the vacuum within the system at about 10- torr during theevaporation process.

The deposited layer of phosphor 16 is polycrystalline and has a densityapproximately equal to the bulk density of the alkali metal halidematerial. The polycrystalline nature of the vacuum evaporated alkalimetal halide material is well known. See, for example, The PhysicalReview, volume 51, No. 5, of Mar. 1, 1937, pages 293298, especiallypages 295-297. Therefore, the X-ray stopping or absorption power of thelayer 16 is substantially improved for a given thickness as compared tothe prior particulated phosphor screens. Thus, the thickness of thelayer 16 can be reduced compared to the prior screens, thereby providingimproved resolution. Moreover, the spherical shape of the layer 16 doesnot interfere with resolution as would be expected to be encountered ifa slab of the alkali halide material were shaped to conform to thespherical substrate 6.

A second method for forming the evaporated pick-up screeen 16 isessentially the same as the first method except that the activatormaterial is not incorporated in the bulk phosphor material 21 to beevaporated. Instead, the activator material 26 is simultaneouslyevaporated from a second crucible 27 which is heated by a separateheater 28. This method provides better control over relative rates ofdeposition of the alkali halide and its activator in order to assure abetter control over the distribution of the activator in the depositedscreen layer 16. As an alternative to employing the second crucible 27,the activator is vaporized in the chamber 17 to form a vapor inequilibrium. The alkali halide material, without the activator, is thenevaporated through the activator vapor and, thus, co-deposited with theactivator vapor on the substrate 6. A third method for forming thescreen layer 16 is essentially the same as the second method except thatthe activator is post evaporated to form a layer upon the previouslydeposited layer of alkali metal halide screen material. The activator isthen diffused into the alkali metal halide screen layer by annealing aspreviously described with regard to the first method.

A fourth method for forming the pick-up screen layer 16 is essentiallythe same as any one of the aforedescribed methods except that thematerials to be evaporated from a heated crucible are instead flashevaporated. More particularly, an evaporation plate 31 is heated by aheater 32 to a temperature well in excess of the evaporation temperatureof the constituents of the material to be evaporated. Pellets 33 of thematerial to be evaporated which in some methods, as aforedescribed,include the alkali metal halide with the activator incorporated therein,and in others of the aforedescribed methods have the activatorseparately evaporated, are dropped upon the plate 31 for flashevaporation. The evaporated material is collected on the substrate 5 toform the pick-up screen layer 16. For the methods wherein the activatoris separately evaporated the proportions of activator and alkali metalhalide, in the resultant deposited layer 16, are controlled bycontrolling the rate at which the separate activator and alkali metalhalide pellets are dropped upon the evaporation plate 31. The resulantscreen layer 16 may be heat treated 01' annealed, as aforedescribed,

to obtain a more uniform distribution of the activator within the alkalimetal halide material and to remove residual plastic deformation.

Still other methods for evaporation of the alkali metal halide materialin vacuum onto the curved face plate 6' include electron beam and laserbeam evaporation methods.

The buffer layer 8 is formed over the pick-up screen layer 16 byevaporating a chemically inert and optically transparent material overthe layer 16 to a thickness less than 10,000 A. and preferably 1000 A.or less. Suitable bufler materials include magnesium oxide, aluminumoxide and lithium fluoride. Such materials are evaporated in vacuum inthe same manner as previously described for evaporation of the pick-upscreen materials. As an alternative, the buffer layer 8 is formed, insome instances, by evaporating the metal constituent such as aluminum ormagnesium and then reacting the deposited metal film with the otherconstituent of the buffer such as oxygen gas, which is introduced intothe vacuum chamber 17, to form the butter layer 8.

The photo-cathode layer 9, as of Cs Sb, is deposited over the bufferlayer 8 by conventional vacuum evaporation methods for forming suchcathodes. Such a method is described in a book entitled, PhotoelectronicMaterials and Devices published by D. Van Nostrand Company, Inc. in 1965at pp. 20020l.

One advantage of the aforedescribed vacuum evaporation methods forforming the pick-up screen 16 and subsequent bufier and photo-cathodelayers 8 and 9, respectively, is that such methods are all performed invacuum such that they lend themselves to a production machine whichperforms the successive steps in vacuum without having to take the partsinto air for performing successive steps in the manufacture.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention can be madewithout departing from the scope thereof it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a method for making an X-ray pick-up screen of improved resolutionand quantum conversion efiiciency for an evacuated X-ray imageintensifier tube the steps of, vaporizing in vacuum an alkali metalhalide material selected from the group consisting of CsI, NaI, LiI, KI,CsBr and RM, condensing in vacuum the vaporized material on a substrateto form on said substrate a layer having a density approximately equalto that of the bulk alkali metal halide material, and incorporating anactivator material into the condensed layer forming an activated X-raysensitive scintillator for the X-ray image tube.

2. The method of claim 1 wherein the step of incorporating saidactivator material into said condensed layer includes the steps of,co-evaporating and co-condensing said activator material and said alkalimetal halide material in vacuum onto said substrate.

3. The method of claim 1 wherein the step of incorporating, saidactivator material into said condensed layer includes the step of,evaporating in vacuum said alkali metal halide material having saidactivator material incorporated therein as an activator for said alkalimetal halide such that the material to be evaporated is preactivatedwith said activator material.

4. The method of claim 1 wherein said alkali metal halide material isCsI.

5. The method of claim 4 wherein said photocathode includes cesium andantimony.

6. The method of claim 1 wherein said alkali metal halide is CsI andsaid activator material is selected from the group consisting of Na andNaI.

7. The method of claim 1 wherein the step of incorporating saidactivator material into said condensed layer includes the step of,coating the surface of said condensed layer with said activator materialand heating said coated layer to dilfuse said activator material intosaid layer.

8. The method of claim 7 wherein the step of coating the surface of saidcondensed layer with said activator material includes the step of,evaporating in vacuum said activator material, and condensing saidevaporated activator material on said condensed layer of said alkalimetal halide material.

9. The method of claim 1 including the step of, annealing said condensedlayer of alkali metal halide material.

10. The method of claim 1 wherein said activator material is selectedfrom the group consisting of TH, NaI, Na, and LiI.

11. The method of claim 1 wherein the substrate is dish-shaped to form adish-shaped scintillator layer.

12. The method of claim 1 including the step of, depositing aphotocathode material overlaying said scintillator which is sensitive tothe X-ray induced scintillations of said scintillator for converting thescintillated image into an electron image to be emitted into theevacuated X-ray image intensifier tube.

13. The method of claim 1 wherein said X-ray pickup screen includes apolycrystalline X-ray sensitive scintillator, and wherein the step ofcondensing the vaporized material includes, condensing the vaporizedmaterial in vacuum on the substrate forming on said substrate apolycrystalline layer, and wherein the step of incorporating saidactivator into said condensed layer includes, incorporating saidactivator into said polycrystalline layer for activating same formingthe activated X-ray sensitive polycrystalline scintillator.

14. The method of claim 1 wherein said activated X- ray sensitivescintillator layer is of a thickness greater than 0.0005 inch.

15. In a method for making an evacuated X-ray image intensifier tube,the steps of; vaporizing in vacuum an alkali metal halide materialselected from the group consisting of CsI, NaI, LiI, KI, CsBr, and RM;condensing in vacuum the vaporized material on a substrate to form apolycrystalline layer on said substrate, said layer having a densityapproximately equal to that of the bulk alkali metal halide material;incorporating an activator material into the condensed layer forming anactivated X-ray sensitive polycystalline scintillator; and disposingsaid activated X-ray sensitive polycrystalline scintillator screenwithin an evacuable envelope of an X-ray intensifier tube for receivingan X-ray image to be intensified.

References Cited UNITED STATES PATENTS 3,023,313 2/1962 De La Mater etal.

117-33.5 X 2,936,246 5/1960 Coghill 117-33.5 X 2,673,816 3/1954 Neuhauset al. 117-33.5 X 2,789,062 4/1957 Cusano et a1. 117-33.5 X 2,798,8237/1957 Harper 117-33.5 2,676,113 4/1954 Jervis 117-33.5 C 2,898,2258/1959 Rychlewski et al. 117-33.5 C 2,903,378 9/1959 Rychlewski 117-33.5C

RALPH S. KENDALL, Primary Examiner US. Cl. X.R.

I UNITED S'I'A'Iiii'i PATENT OFFICE C'ER'REFICA'IE OF CORRECTION recentNo. C v 3 795, 531 Dated March 5, 1974 Inventofl William E. Spicer It iscertified that error appears in the above-identified patent and thatsaid Letters Patent are hereby corrected as shown below:

Column 1, line 33, After "application." insert -This application claimsa method of fabrication,

the device of the invention is claimed in a division hereof, Serial No.281,905; filed 18 August 1972 Claim 1, line 10, Change "forming" to --toform-.

Claim 11, line 2, After "dish-shaped" insert -so as--.

Claim 12, line 3, Change "which is" to said material being.

Claim 13, line 5, After "substrate", l st occ insert line 9, Change"forming the" to so as to 7 form an--.

Claim 15, line 9, Change "forming" to -to form--.

Claim 3, lines l-Z, A After "incorporating" delete Signed and sealedthis 29th day of October 1974.

(sEAr) Attest:

Mccor" n. GIBSON JR. c. MARSHALL DANN attesting Officer Commissioner ofPatents

